Chromium surfaces are used in various applications such as a decorative metal finish for plastic parts in automotive industry and sanitary equipment. The chromium surface is usually the outer surface of the substrate and obtained by electroplating a chromium layer from plating bath compositions comprising either Cr(III) ions, Cr(VI) ions or both.
In some applications, at least one other metal or metal alloy layer is located between said chromium layer and the substrate. The at least one metal or metal alloy layer is selected from one or more of nickel layer, nickel alloy layer, copper layer and copper alloy layer.
The chromium layer usually comprises micro-cracks after plating or (thermal) annealing, or pores created by an underlying micro-porous nickel layer. Hence, also the layer material(s) between the chromium layer and the substrate are exposed to the environment. The chromium oxide layer formed on the outer surface of the chromium layer protects said outer surface of the chromium layer from corrosion but not the underlying layer(s). Such multilayer assemblies comprising a chromium layer as the outermost layer are for example disclosed in US 2012/0052319 A1.
The resulting chromium surface is usually very shiny and fulfils aesthetic requirements. However, in some applications one or more layers and/or types of a lacquer are applied to said chromium surface.
Typical lacquers applied to chromium surfaces comprise e.g. polyurethane-based lacquers and epoxy-based lacquers.
The adhesion between an untreated chromium surface and a lacquer is not sufficient.
Accordingly, different methods to increase the adhesion between a chromium surface and a lacquer are known in the art:
Functional organophosphonic acid esters as adhesion promoting agents for metal surfaces are disclosed in U.S. Pat. No. 4,048,374 A. The metal surface is immersed in a solution comprising an organophosphonic acid ester without applying an external electrical current to the metal surface. The adhesion between a metal surface and a lacquer is not sufficiently improved when the metal surface to be treated is a chromium surface (Example 2, present invention).
An adhesion promoting agent for lacquers on a metal surface is also disclosed in DE 100 51 486 A1. Said adhesion promoting agent contains at least two compounds each comprising a functional group selected from COOH, HSO3, HSO4, (OH)2PO, (OH)2PO2, (OH)(OR2)PO and (OH)(OR2)PO2 wherein R2 is selected from linear and branched C1 to C10 alkyl. Application of an external electrical current to the metal surface during such treatment is not disclosed. The adhesion between a chromium surface and a lacquer is not sufficiently improved by said method (Example 2, present invention).
An anodic treatment of metal surfaces with an aqueous solution comprising a compound having hydrophobic carbon-chains with hydrophilic anionic functional groups is disclosed in EP 2 186 928 A1. Residues creating a foggy appearance remain on the chromium surface after the anodic treatment after rinsing with water (Example 3, present invention). Hence, said method is not suitable to increase the adhesion of a chromium surface and maintain the optical properties of said chromium surface, i.e. the shiny and decorative optical appearance when an optically transparent lacquer is deposited onto the treated chromium surface.